Systematic Updating Of Records Depending On Amount Of Records In Another Table

The problem concerns 2 tables, cart_products and cart_shoppingcart.

Cart_products contains all the product data (eg, prices, product's key name, etc), but most importantly, it contains the quantity of stock (prod_quantity) for each item.

cart_shoppingcart contains all of the user's items in their shoppingcart, including those that are part of an (customer) order; these have the order_number to differentiate from those that aren't in any order (their Order_number is 'no_order', the others have the order number).

Now, what I want to do is this:

For every item in the order (ie, every entry in cart_shoppingcart that matches the username and order number), take the quantity (x) the customer is ordering, then take the quantity of stock remaining (y), then subtract x from y to create the new stock level (z) and update the appropriate record in cart_products with z (all this with SQl preferably).

I was thinking of using a Do while loop, but realised that it wouldn't work as I'd have to use several SQL statements and then I'd run into a problem the minute ASP reads "objrec.movenext".